This invention relates to a setting time indicator for acrylic bone cement. More particularly the acrylic bone cement of the invention indicates its setting point in situ by a change in its color, which change can be visually recognized.
Bone cements find wide usage in a variety of applications. For instance, they are used for cementing orthopedic implants in place, for the anchoring of endoprosthesis of the joints, for filling voids in bone, in the treatment of skull defects, and for the performance of spinal fusion. These cements are typically polymeric materials and more particularly acrylic polymers and the surgeon usually mixes the interactive components to make the cement at an appropriate stage during the surgical procedure.
Typically, the components of the bone cement comprise a powdered homopolymer or copolymer of methyl methacrylates, alkyl methacrylates and/or styrene and a suitable liquid monomer. The liquid monomer consists of esters of acrylic or methacrylic acid for example methyl methacrylate. The liquid monomer is typically provided in a glass ampoule. To accelerate the polymerization of the bone cement, a catalyst system may also be used. The catalyst, if present, is in the form of a redox catalyst system, usually containing an organic peroxy compound, such as dibenzoyl peroxide, plus a reducing component, such as p-toluidine. N, N-dimethylparatoluidine (DMPT) can also be used as a polymerization accelerator and hydroquinone (HQ) can be used as a stabilizer. The DMPT and HQ may be included with the liquid monomer. A radiopacifier such as barium sulphate may also be included.
After the bone is prepared the liquid and powdered components of the bone cement are mixed. The setting time is one of the most important characteristics of acrylic bone cement. The setting time is the point after mixing at which the cement is hardened. Although all bone cement manufacturers indicate the setting profile in their product inserts, the actual setting properties in an operating room (OR) may vary significantly due to different environmental conditions such as temperature, storage conditions and mixing methods. Therefore, it is sometimes difficult for cement users to predict when the cement sets in situ.
Surgeons or nurses have sometimes used excess cement to determine the setting point of the implanted cement by placing the cement on a surface in the OR or by holding it in their hands. The OR personnel use the time when the excess cement gets warm and hard to determine the setting point of the implanted cement. This assumes that the implanted cement behaves the same as the excess cement. Because of the different environmental factors, the setting time of the “bench” cement may be significantly different to that of the implanted cement. While it may be possible to determine the setting point in situ by monitoring the temperature rise of cemented implants during a cement setting process, such is difficult and inaccurate. It would be advantageous to have an acrylic bone cement available which indicates its setting point in situ.
One advantage for surgeons is that the recognition of the setting point of bone cement in situ prevents early loading of the joint, which may cause migration of implants. It may also eliminate unnecessary surgical site exposure time should the surgeon overestimate the setting time. Therefore, development of a cement that is able to indicate its setting point in situ would benefit both bone cement users and patients. In addition colored cements may help surgeons easily distinguish the bone cement from the surrounding tissues especially during revision surgery.
The setting process of acrylic bone cement is a free-radical polymerization reaction of methyl methacrylate (MMA) monomer. The bone cement sets when most of MMA monomer is converted to polymethyl methacrylate (PMMA) polymer through free-radical polymerization. By monitoring the free-radical polymerization of MMA monomer, one can determine the setting point of bone cement. Based on this rationale, the cement of the present invention uses color change to visually indicate the setting point in situ and also leaves a colored cement for visual identification.
Two color pigments, β-carotene (pro-vitamin A) and FDC blue No. 2 Lake, were used to formulate this colored cement. Carotene is a natural product that exist in plant and fruits and is a major source of Vitamin A. As an orange-red powder, it is soluble in organic solvents such as methyl methacrylate and gives a yellow-orange color. Carotene belongs to the category “exempt from certification” classified by FDA and is widely used in food industry as GRAS (Generally Regarded as Safe).
FDC blue No. 2 Aluminum Lake is a color additive that has been approved for use in acrylic bone cement in an amount of up to 0.1% (w/w). Methylene blue powder also may be used as we as chlorophyll which changes from light. It is insoluble in most solvents including water and methyl methacrylate. It has a good thermal stability and has been used in commercial bone cement products. It is supplied as a fine powder from Sensient Inc. of St. Louis.
As discussed above, acrylic bone cements are made from combining a powder polymeric component and a liquid monomer component and a polymerization initiator. One well known system is manufactured and sold by Howmedica Osteonics Corp. as Simplex® P bone cement. Heretofore, none of these types of systems have used color to indicate setting time.
U.S. Pat. No. 6,017,983 (Gilleo) relates to the use of a diazo dye that is believed to form a salt or complex with acid anhydrides, which acts as a color indicator for particular anhydride/epoxy resin thermoset adhesives. The resulting salt or complex is reported to produce a chromophoric shift in the dye which is indicative of the amount of acid anhydride present, and hence, the degree of cure. As the epoxy resin cures, the amount of acid anhydride diminishes, thus, producing a color change. This system appears to be limited to acid anhydride hardeners used to cure epoxy resins.
U.S. Publication No. 2003/0139488 (Wojciok) relates to a (meth) acrylate composition comprising a (meth) acrylate component; and a dye substantially dissolved in the (meth) acrylate component which imparts a first color to the (meth) acrylate component, wherein upon curing, a resultant cured composition has a second color. Preferably, upon curing, the resultant cured composition is substantially free of the first color.